A novel configuration for a closed-circuit PRO (CPRO) system is described. The CPRO system enables fluid flow in both the draw and feed channels using gravitational potential energy. The new PRO system is designed to store gravitational potential energy derived from input thermal energy for use in fluid circulation, thereby bypassing the need for circulation pumps. As a result the novel PRO system can recover a significant amount of the 66% of energy typically unavailable for external use. The new PRO system is self-cleaning with all salt (in the draw solution, having leaked through the membrane) washed into the distillation unit. Circulation of distilled water is accomplished using stored gravitational potential energy. Circulation of the saline solution is accomplished using a gravitationally powered volumetric mass transfer process without significant loss of internal pressure. A laboratory test device was constructed and operated. It's top performance is evaluated at a recovery of 67.0% of the energy potential.